This application claims the priority of German patent document 100 35 530.7, filed Jul. 21, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for computing the coefficients of a filter and for filtering radar source data of the distance Doppler matrices of two adjacent receiving channels, by means of Space-Time Adaptive Processing.
Radar source data consist of a mixture of signals (for example, echoes of moving and stationary targets) and interferences (for example, noise). As a rule, interference is to be suppressed when evaluating these radar source data, so that the signals can easily be detected.
To improve the detection of moving targets, for example, in synthetic aperture radar (xe2x80x9cSARxe2x80x9d) images, it is necessary to suppress interference as well as signals which originate from stationary targets. A suitable method for this purpose is the Space-Time Adaptive Processing (xe2x80x9cSTAPxe2x80x9d) . In known processes of this type, the radar source data are as a rule filtered in the time domain (xe2x80x9cSpecial Issue on Space-Time Adaptive Processing (STAP)xe2x80x9d, Electronics and Communication Engineering Journal, Volume 11, 1999, February, No. 1, ISSN 0954-0695). Since the determination of the filtering coefficients and filtering in the time domain require high computing expenditures, in practical applications of these methods, the number of filtering coefficients must be relatively limited. This results in a limited filtering quality and thus in a stationary target suppression which is not optimal and which, furthermore, operates satisfactorily only in the range of the major lobe of the antenna.
In German Patent Document DE 100 12 411, which is not prior art, a method is introduced in which the filtering coefficients and the filtering are determined in the frequency domain, so that the number of computing operations per matrix element is limited to a few. In this manner, an optical filtering characteristic is nevertheless implemented, so that the stationary target suppression also functions in the range of the minor lobes of the antenna.
However, the known methods of STAP processing have a common deficiency in that they can either process data of individual adjacent channels or data from sum and difference channels (xcexa3/xcex94 data), but not both types of data.
One object of the present invention is to provide a novel method for STAP processing which has an expanded functionality in comparison to known prior art methods.
This and other objects and advantages are achieved by the method according to the invention, with which it is possible, within the scope of a STAP processing, to process data of two-channel systems, which are present either as data of individual channels (L/R, left/right) or as data of sum and difference channels (xcexa3/xcex94 data), in the same manner. For this purpose, the computing of the filtering coefficients and the filtering preferably take place in the frequency domain. Advantageously, for determining the filtering coefficients, both the auto-correlations and cross-correlations of the signals of both receiving channels are taken into account.
In an advantageous embodiment of the method according to the invention, filtering in the frequency domain takes place essentially in three steps. In a first step, the auto- and cross-correlations of both input channels are determined, in a very simple manner, using algorithms which are generally known to a person skilled in the art. In the second step, the filtering coefficients xcex1(i) and xcex2(i) of the STAP filter are determined; and, subsequently, in a third step, the input signals {overscore (X)}1 and {overscore (X)}2 are filtered in the frequency domain in order to arrive at a result matrix Yclutterfree which represents a range/Doppler matrix in which the echoes of stationary targets are contained in a suppressed state and the echoes of moving targets are contained in a coherently intensified state.
A representative embodiment of the method according to the invention which is advantageous for the implementation will be discussed in the following. In this case, X1(i,j) and X2(i,j) exist with 1xe2x89xa6ixe2x89xa6NDZ and 1xe2x89xa6jxe2x89xa6NRG, wherein X1(i,j) is the complex range/Doppler matrix of the first channel (for example, L or xcexa3) of a coherent radar system, and X2(i,j) is the complex range/Doppler matrix of the second channel (for example, R or xcex94). Furthermore, NDZ is the number of Doppler cells and NRG is the number of range gates (distance gates). The desired complex range/Doppler matrix to be determined by means of as few computing operations as possible will be Yclutterfree (i, j)
In the first step of the method according to the invention, the auto- and cross-relations of both input channels are determined according to Equations 1 to 3. Here, r11 indicates the auto-correlation of the first channel; r22 indicates the auto-correlation of the second channel, and r12 indicates the cross-correlation of both channels.                                           r            11                    ⁡                      (            i            )                          =                              ∑                          j              =              1                                      N              RG                                ⁢                                                    X                1                            ⁡                              (                                  i                  ,                  j                                )                                      ·                                          X                1                *                            ⁡                              (                                  i                  ,                  j                                )                                                                        Equation  1                                                      r            22                    ⁡                      (            i            )                          =                              ∑                          j              =              1                                      N              RG                                ⁢                                                    X                2                            ⁡                              (                                  i                  ,                  j                                )                                      ·                                          X                2                *                            ⁡                              (                                  i                  ,                  j                                )                                                                        Equation  2                                                      r            12                    ⁡                      (            i            )                          =                              ∑                          j              =              1                                      N              RG                                ⁢                                                    X                1                            ⁡                              (                                  i                  ,                  j                                )                                      ·                                          X                2                *                            ⁡                              (                                  i                  ,                  j                                )                                                                        Equation  3            
with 1xe2x89xa6ixe2x89xa6NDZ.
Advantageously, the values of the auto- or cross-correlations determined from the two input channels may be subjected to an additional processing before the determination of the filtering coefficients xcex1(i) and xcex2(i) of the STAP filter.
In this case, it is, for example, advantageous to determine which of the two channels has the better signal-to-noise ratio (S/N). A possible method for this purpose is indicated in Equation 4.                                           max            ⁡                          (                                                                    r                    11                                    ⁡                                      (                    i                    )                                                  |                                  1                  ≤                  i                  ≤                                      N                    DZ                                                              )                                                          ∑                              i                =                1                                            N                DZ                                      ⁢                                          r                11                            ⁡                              (                i                )                                                     greater than                               max            ⁡                          (                                                                    r                    22                                    ⁡                                      (                    i                    )                                                  |                                  1                  ≤                  i                  ≤                                      N                    DZ                                                              )                                                          ∑                              i                =                1                                            N                DZ                                      ⁢                                          r                22                            ⁡                              (                i                )                                                                        Equation        ⁢                  xe2x80x83                ⁢        4            
If this equation is true, the first channel (corresponding to {overscore (X)}1 with r11) has a better signal-to-noise ratio; otherwise, the second channel (corresponding to {overscore (X)}2 with r22)
Following this determination, advantageously, as a function of which of the two channels {overscore (X)}1 or {overscore (X)}2 has a better signal-to-noise ratio (S/N), a defined pair of equations can be used to determine the filtering coefficients xcex1(i) and xcex2(i) of the STAP filter. In this case, the selection rule is profitably designed such that, in the event that the channel {overscore (X)}1 has a better signal-to-noise ratio (S/N), the pair of equations 5 and 6 is selected, but otherwise the pair of equations 7 and 8 is selected.
a(i)=xe2x88x92(2l+1)xe2x80x83xe2x80x83Equation 5
                              b          ⁡                      (            i            )                          =                              ∑                          k              =                                                N                  DZ                                -                l                                                                    N                DZ                            +              l                                ⁢                                                    r                12                            ⁡                              (                                                                            (                                              i                        +                        k                        -                        1                                            )                                        ⁢                    mod                    ⁢                                          xe2x80x83                                        ⁢                                          N                      DZ                                                        +                  1                                )                                                                    r                22                            ⁡                              (                                                                            (                                              i                        +                        k                        -                        1                                            )                                        ⁢                    mod                    ⁢                                          xe2x80x83                                        ⁢                                          N                      DZ                                                        +                  1                                )                                                                        Equation        ⁢                  xe2x80x83                ⁢        6            
for 1xe2x89xa6ixe2x89xa6NDZ, and l=0, 1, 2, . . . ;
                              a          ⁡                      (            i            )                          =                              ∑                          k              =                                                N                  DZ                                -                l                                                                    N                DZ                            +              l                                ⁢                                                    r                12                *                            ⁡                              (                                                                            (                                              i                        +                        k                        -                        1                                            )                                        ⁢                    mod                    ⁢                                          xe2x80x83                                        ⁢                                          N                      DZ                                                        +                  1                                )                                                                    r                11                            ⁡                              (                                                                            (                                              i                        +                        k                        -                        1                                            )                                        ⁢                    mod                    ⁢                                          xe2x80x83                                        ⁢                                          N                      DZ                                                        +                  1                                )                                                                        Equation        ⁢                  xe2x80x83                ⁢        7            xe2x80x83b(i)=xe2x88x92(2l+1)xe2x80x83xe2x80x83Equation 8
for 1xe2x89xa6ixe2x89xa6NDZ, and l=0, 1, 2, . . . ;
According to Equations 6,7 the mean value of the correlation values of 2l+1, adjacent Doppler cells are used to determine the filtering coefficients a(i), b(i) . However, l=0 is also possible, i.e., only the correlation values of one Doppler cell (Doppler cell i) are taken into account.
Advantageously, the method according to the invention can now be modified such that a deviation from the selection rule takes place when the channel with the poorer signal-to-noise ratio S/N) clearly has a lower signal output in comparison to the better channel (for example, at least 20 dB less signal output). As a result, it can be avoided that, because of an extremely low factor in the divisor (r11 or r22), an unintended elevation of a filtering coefficient (b(i) or a(i)) takes place. In the event of this exception, the respective other pair of equations will be used.
In a particularly advantageous manner, the filtering coefficients can be scaled after their determination. As a result, a homogeneous filtering effect is achievable via all Doppler frequencies. In a particularly advantageous manner, this is possible by means of Equations 9 and 10:                               α          ⁡                      (            i            )                          =                              a            ⁡                          (              i              )                                                                                            a                  ⁡                                      (                    i                    )                                                  ·                                                      a                    *                                    ⁡                                      (                    i                    )                                                              +                                                b                  ⁡                                      (                    i                    )                                                  ·                                                      b                    *                                    ⁡                                      (                    i                    )                                                                                                          Equation  9                                          β          ⁡                      (            i            )                          =                              b            ⁡                          (              i              )                                                                                            a                  ⁡                                      (                    i                    )                                                  ·                                                      a                    *                                    ⁡                                      (                    i                    )                                                              +                                                b                  ⁡                                      (                    i                    )                                                  ·                                                      b                    *                                    ⁡                                      (                    i                    )                                                                                                          Equation  10            
with 1xe2x89xa6ixe2x89xa6NDZ.
In order to save computing performance, it is, however, also possible to eliminate the scaling, and to use the filtering coefficients a (i) or b (i) directly for the filtering coefficients xcex1(i) and xcex2(i).
In a final step of the method according to the invention, the filtering coefficients xcex1(i) and xcex2(i) are used to process the complex range/Doppler matrices X1(i,j) and X2(i,j). As a result, according to Equation 11, the result matrix Yclutterfree is obtained which represents the wanted range/Doppler matrix in which the echoes of stationary targets are suppressed and the echoes of moving targets are contained in a coherently intensified state.
Yclutterfree(i,j)=xcex1(i)xc2x7X1(i,j)+xcex2(i)xc2x7X2(i,j)
with 1xe2x89xa6ixe2x89xa6NDZ and 1xe2x89xa6jxe2x89xa6NRG.
As a result of the method for a STAP processing according to the invention, which, as it were, is capable of processing individual channels (left/right) as well as sum and difference channels, optimal filtering characteristics can be implemented. It is therefore possible to suppress stationary targets also in the range of minor lobes of the antenna.